Repeated use of amphetamine and cocaine produces sensitization, which is thought to underlie craving and addiction. Over the last decade, several lines of research have shown that psychostimulants act by dramatically increasing dopamine levels in the nucleus accumbens. This is due to action at the presynaptic terminals of ventral tegmental area dopamine neurons. These dopamine neurons connect mainly to medium-spiny GABA neurons, which give rise to most intrinsic synapses as well as to the efferent projections of the nucleus accumbens. The present proposal seeks to examine this synaptic substrate of psychostimulant action at the level of single synapses focusing on synaptic plastic changes that may underlie sensitization. Using postnatal cell cultures of identified mesoaccumbens dopamine neurons and nucleus accumbens GABA neurons, the following issues will be addressed: * The role of glutamate as a cotransmitter in dopamine neurons. Sensitization can be blocked by infusion & glutamate antagonists in the ventral tegmental area. New evidence suggests that glutamate is a co- transmitter in dopamine neurons. Microcultures of single dopamine neurons will be studied with whole-cell patch recording to elucidate the interplay between dopamine and glutamate at individual synapses. Co- localized glutamate may also have neurotoxic effects leading to long-term changes in synaptic function that will be examined in culture using fluorescence imaging of free radical production. * Intrinsic nucleus accumbens synapses as the target of psychostimulant action. Since the synapses of medium-spiny GABA neurons mediate most accumbens information processing, how dopamine modulates-these GABA synapses may be important in psychostimulant action. Electrophysiological analysis and receptor imaging will be used to investigate the modulatory action of dopamine at single GABA synapses. * Psychostimulant modulation of dopamine synapses. Since amphetamine reduces vesicular dopamine content, it should attenuate activity- dependent release and at the same time tonically increase synaptic dopamine. Amphetamine should also selectively release dopamine and not glutamate. Cocaine would be expected to act similarly. These hypotheses will be tested in microcultures containing single dopamine neurons and in co-cultures with nucleus accumbens neurons. The possibility that neurotoxic effects of psychostimulants contribute to long-term plastic changes in the accumbens synapses will also be examined. Results from this work should provide a better understanding of how psychostimulants act at the synaptic level to induce sensitization, and provide the foundations for the development of new pharmacological interventions targeted at reversing sensitization and thus addiction.